Malaria is a vector-borne infectious disease caused by protozoan parasites and is widespread in tropical and subtropical regions, including parts of the Americas, Asia and Africa. Of the four Plasmodium parasite species that can infect humans (P. falciparum, P. vivax, P. ovale and P. malariae), the most serious forms of the disease are caused by P. falciparum and P. vivax. Of the approximately 515 million people infected yearly, between one and three million people, the majority of whom are young children in Sub-Saharan Africa, die from the disease. The current armament of approved anti-malarial drugs is limited to only a few targets within the human malaria parasite, and growing widespread resistance to current drugs is prompting the development of new antimalarial agents that have new biological targets.
Pyrimidines are required for the biosynthesis of DNA, RNA, glycoproteins, and phospholipids. Most organisms possess both a salvage and de novo pyrimidine biosynthetic pathway; however, the human malaria parasite P. falciparum cannot salvage preformed pyrimidine bases or nucleosides from its host and thus is entirely dependent on de novo biosynthesis [1, 2]. Dihydroorotate dehydrogenase (DHODH) is a flavoenzyme that catalyzes the oxidation of L-dihydroorotate (L-DHO) to orotate as part of the fourth sequential step of the de novo pyrimidine biosynthetic pathway [3-6].
Although the function of DHODH is conserved among all organisms, the enzyme family can be separated into two broad classes based upon sequence homology that correlates with cellular localization and preference for electron acceptors [6-9]. Both classes of enzyme perform a two-step reaction that most likely proceeds through a ping-pong mechanism [10-13]. Type 1 DHODH, as found in Gram-positive bacteria and the anaerobic yeast Saccharomyces cerevisiae, is located in the cytosol and utilizes fumarate or NAD as an electron acceptor to re-oxidize the flavin (FMN) prosthetic group in the second half reaction of the redox process [4, 14-17]. Type 2 DHODHs, generally found in eukaryotes and some Gram-negative bacteria [18-21], are membrane-bound and utilize quinones located in biological membranes as a final electron acceptor [22]. Both humans and all characterized members of the genus Plasmodia utilize a Type 2 DHODH for de novo pyrimidine biosynthesis [23-27]. Specifically, eukaryotic Type 2 DHODHs are localized to the inner mitochondrial space and utilize the respiratory chain quinone coenzyme Qn (CoQn), where the length of the isoprenoid unit (n) is variable, as a final electron acceptor.
The continued development of novel anti-malarial chemotherapies, particularly those aimed at new pathways, is necessary for the successful treatment of malaria as resistance to presently utilized drugs becomes more widespread. PfDHODH is likely to be an essential enzyme because the malaria parasite must rely on de novo biosynthesis for its metabolic needs. Identification of compounds able to inhibit pfDHODH could facilitate the development of effective antimalarials.